It is commonly known that certain luminescent phosphors have the ability to store energy which strikes them in the proper form. In particular, it has been found that some phosphors have the ability to store energy dependent upon the amount of gamma and X radiation which has impinged upon the phosphor. This phenomenon has been put to use in the manufacture of dosimeters which record the amount of high energy radiation to which a worker has been exposed. Such dosimeters are commonly used in and around nuclear facilities.
Before this invention, it has been common to release the luminescent energy stored within the luminescent phosphors by using a heat source. Upon heating, the phosphor emits a beam of light or other electromagnetic radiation which is detected and the intensity and duration of the beam is measured so that the amount of radiation exposure can be determined.
Stimulation of such luminescent phosphors has typically been by heat sources and is commonly called thermoluminescence. Thermoluminescent stimulation occurs using direct heating by contact with a hot object, heating by using an infrared beam produced by incandescence or by using a laser which heats the phosphor and thereby releases the luminescence stored within, as shown in U.S. Pat. No. 3,729,630 to Yamashita et al. Such thermoluminescent stimulation techniques are relatively slow with the fastest response time indicated by Yamashita as being about one half second. Although this may appear to be a fast response time in the technical area of dosimeters, other applications such as real time imaging, require that stimulation occur much more quickly and until this time rapid stimulation occurring in less than one microsecond has not been possible.
Another problem encountered in stimulating luminescent phosphors is the relatively poor signal-to-noise ratio which can be achieved with current stimulation methods and apparatus. When a luminescent phosphor is exposed to a heating element or heating laser, it takes a substantial amount of time in order to heat and bring about the emission from the luminescent material. Since the emission occurs over a relatively long period of time and at a relatively low level of luminous intensity, the background electronic noise is a sizeable component of the total measurement. This poor signal-to-noise ratio makes it difficult to read with precision the amount of luminescent energy which is released from the phosphor.
Prior art phosphor stimulation techniques have also been limited because it is difficult to focus a stimulating infrared beam to a size much smaller than 50 micrometers in diameter without employing very costly and sophisticated optical equipment. This increases the amount of luminescent phosphor which must be used for any application and also increases the amount of energy necessary to stimulate that phosphor. Small size units of phosphor are desirable when good resolution imaging is desired.
The current invention has been developed to solve the problems discussed above and other problems.